1. FIELD OF THE INVENTION
This invention relates to a blanket for an extended nip press. More particularly, this invention relates to a vented blanket for an extended nip press which includes a shoe and a felt.
2. INFORMATION DISCLOSURE STATEMENT
With the introduction, by Beloit Corporation, of the so-called extended nip press, the art of papermaking has been revolutionized.
More specifically, an extended nip press includes a backing roll and a shoe defining a concave surface for cooperation with the backing roll. A band-shaped blanket (or belt) together with a felt and a paper web extend through an extended nip defined between the concave surface and the backing roll. By the provision of lubricant between the blanket and the concave surface, the residence time of the web within the nip is increased resulting in the removal of greater quantities of water from the web than is possible with a conventional nip defined by cooperating rolls.
In view of the very high pressures involved in extended nip presses (6000 psi), it is evident that the blanket is subjected to enormous compressional forces during passage through the extended nip. However, in order to facilitate drainage of increased quantities of water removed from the web to the felt during transit through the extended nip, it has been found advantageous to provide blind drilled holes or drainage channels (grooves) in the upper surface of the blanket. The arrangement is such that as the vented surface of the blanket comes into contact with the felt during passage through the extended nip, water flows from the felt into the blind drilled holes or grooves and away from the extended nip.
Preferably, in the case of grooves, the grooves are closely spaced relative to each other in order to provide maximum drainage from the extended nip. Furthermore, the grooves should have a width and depth sufficient to allow adequate flow of water therethrough. Unfortunately, the provision of closely-spaced grooves of sufficient depth and width weakens the structure of the blanket and this has resulted in collapsing or "barreling" of the grooves after prolonged usage of the prior art blankets.
In an attempt to overcome the aforementioned problems of barreling, or collapsing of the aforementioned blankets which has resulted in impairment of the drainage capabilities of these blankets, various proposals have been set forth in an attempt to provide a blanket having sufficient rigidity to inhibit closing of the grooves while maintaining sufficient flexibility of the blanket during transit through the extended nip.
U.S. Pat. No. 4,552,620 to Adams, assigned to Beloit Corporation, teaches an endless impervious belt for an extended nip having a woven fiber base impregnated with a urethane coating. The belt is formed by looping an endless scrim blanket around driving rollers providing a traveling run which is sprayed with a two-component polyurethane coating which quickly gels to accommodate building up of a layer of the desired thickness by controlling the fluid flow and speed of the traveling run. The resultant belt has a durometer hardness within the range 70-90 on the Shore A scale and column 4, lines 38-40 teach milling the surface 20 to form grooves 26 extending longitudinally around the belt.
U.S. Pat. No. 4,496,429 discloses an endless band which is impermeable to water. Column 2, lines 51-53 teach grooves for receiving water from the felt in the pressed surface of the V-bands or V-belts.
U.S. Pat. No. 4,482,430 to Majaniemi teaches a bearing blanket for an extended nip press in which a plurality of parallel spaced grooves 14 (shown in FIG. 6) are defined by the blanket and come into physical contact with a felt 8. However, as shown in FIGS. 4 and 6, the belt includes a woven base layer 15.
In the prior art, various proposals have been set forth which include the provision of a woven base fabric having applied thereto, respective coatings of polyurethane or the like. However, the resultant laminates have suffered from the problem of delamination after extensive use of the blanket. Because of the aforementioned, extremely high pressures involved in extended nip presses, there has existed a tendency for the laminated coatings to delaminate relative to the woven base.
The present invention seeks to overcome the aforementioned problem of delamination by providing a monocoque fiber-reinforced polyurethane blanket in which the reinforcing fibers are non-woven or partially oriented in the machine direction. The fibers therefore become randomly oriented throughout the thickness of the blanket structure.
Throughout the specification, the term "monocoque" means a composite, unitary construction wherein at least some of the fibers break through the surface of the blanket rather than the conventional blankets which include a plurality of laminates.
The arrangement is such that many of the randomly oriented fibers extend from within the blanket to the respective surfaces thereof, thereby imparting to the blanket increased strength, and an improved modulus of elasticity while avoiding the problem of delamination associated with fully woven base blankets.
Therefore, it is a primary object of the present invention to provide a blanket which overcomes the aforementioned problems associated with the prior art blankets and which contributes a significant improvement to the paper pressing art.
Another object of the present invention is the provision of a blanket which is fabricated from fiber-reinforced polyurethane for inhibiting delamination of the body during use of the blanket.
Another object of the present invention is the provision of a blanket for an extended nip press defining a plurality of water receiving blind drilled holes.
Another object of the present invention is the provision of a blanket for an extended nip press which is impervious to the flow of lubricant disposed between the shoe and the inner surface of the blanket body.
Another object of the present invention is the provision of a blanket for an extended nip press having a uniform thickness in the load zone and with grooves that are parallel and spaced relative to each other.
Another object of the present invention is the provision of a blanket body which is fabricated from a polyurethane having fibers randomly oriented therein.
Another object of the present invention is the provision of a blanket for an extended nip press in which the fibers impart reinforcement to the polyurethane such that the blanket body exhibits anisotropic properties.
Another object of the present invention is the provision of a blanket for an extended nip press in which the anisotropic properties of the body permit flexure of the band in a machine direction during passage through the extended nip press while inhibiting closing of the grooves by cross-machine direction flexure of the band thereby maintaining the drainage capabilities of the grooves.
Another object of the present invention is the provision of a method for fabricating a blanket for an extended nip press which includes saturating a fiber mat with polyurethane, wrapping the mat around a smooth mandrel, heating the wrapped mandrel so that the polyurethane gels, further heating the wrapped mandrel to cure the polyurethane, cooling the cured mat and grinding the same to a uniform thickness and then grooving the outer surface of the blanket while the blanket is still supported on the mandrel and thereafter removing the finished blanket from the mandrel.
Another object of the present invention is the provision of a method of fabricating a blanket for an extended nip press in which the fiber mat is non-woven.
Another object of the present invention is the provision of a method of fabricating a blanket for an extended nip press which includes spirally wrapping a saturated mat around a mandrel and overlapping the same such that the mat attains a thickness which is greater than the required thickness of the finished blanket.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter and from the annexed drawings.